1. Field of the Invention
The present invention relates to an injection molding machine in which a screw moves along guide members on a frame, and more particularly, to an injection molding machine configured so that a reactive force produced during material injection can be prevented from affecting the guide members and the screw can be moved smoothly.
2. Description of the Related Art
An injection molding machine comprises a cylinder and screw. As the screw advances in the cylinder, a material (e.g., molten resin) is injected from the cylinder into a die set. To this end, the injection molding machine comprises a screw advancement-retraction mechanism, which moves the screw longitudinally in the cylinder, and a screw rotation mechanism, which rotates the screw in the cylinder. The screw rotation mechanism introduces the material (e.g., resin pellets) into the cylinder and kneads and melts the introduced material.
An example of a conventional injection molding machine is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-80559 (Patent Document 1). This injection molding machine comprises a middle plate located behind a screw, a bearing attached to the middle plate and supporting the rear end portion of the screw, and a motor that rotates the screw. The middle plate is supported on a slide base (moving base) below it by linear guide means. The middle plate is moved longitudinally by a linear motion mechanism, such as a ball screw.
In the conventional injection molding machine described above, the rear end portion of the screw and the motor for rotating the screw are mounted on the middle plate. The middle plate moves longitudinally on the slide base. In this injection molding machine, the position where the motor and the rear end portion of the screw are mounted on the middle plate is at a great vertical distance from the position of the lower part of the middle plate supported on the slide base. When the middle plate is longitudinally moved by the linear motion mechanism, therefore, a longitudinal moment acts on it, thereby causing the middle plate to undergo longitudinal deflection. Thus, the movement of the screw may become unstable in some cases.
In order to solve this problem, an injection molding machine has been proposed that comprises a frame, a pair of lateral members (connection plates) located beside the frame, a screw advancement-retraction mechanism behind the frame, a screw rotation mechanism between the lateral members, and guide members (linear guides) mounted on the lateral members. In this injection molding machine, the guide members and screw rotation mechanism are not substantially separated in the vertical direction, so that the movement of the screw is steady.
If a reactive force produced when the material is injected into the die set acts on the frame through the screw, in the injection molding machine constructed in this manner, the lateral members may be elongated in some cases. Thereupon, the guide members on the lateral members are stressed and bent, or their parallelism is reduced. In such a case, the movement of the screw rotation mechanism becomes so unstable that the screw cannot reliably move along its axis.